1. Field of the Invention
The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material capable of producing images with excellent preservation stability, comprising as the main components a leuco dye serving as a coloring agent and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
2. Discussion of Background
There are conventionally proposed various recording materials which utilize the coloring reaction between a colorless or light-colored electron donor type dye such as a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat or pressure thereto when brought into contact with the leuco dye.
A thermosensitive recording material, one of the above-mentioned recording materials, is proposed as disclosed in Japanese Patent Publications 43-4160 and 45- 14039 and Japanese Laid-Open Patent Application 48-27736. Such a thermosensitive recording material is usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder because it has the advantages that complicated processes such as development and image-fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low.
In such a thermosensitive recording material, colorless or light-colored leuco dyes having a lactone, lactam, or a spiropyran ring are used as coloring dyes, and organic acids or phenols are conventionally employed as color developers. The thermosensitive recording material using the above-mentioned leuco dye and color developer is widely used for practical use because the produced images have high image density, with the whiteness of the background maintained high.
In line with the increase of a demand for the thermosensitive recording system, the requirements for improved preservability of the images recorded on the thermosensitive recording material with respect to the resistance to chemicals, light and heat are increased. Therefore, the development of a recording material capable of meeting the above-mentioned requirements is intensively desired.
To improve the preservation stability of the recorded images on the thermosensitive recording material, it is proposed to use as the color developer with high reliability a phenolsulfonic acid compound, as disclosed in Japanese Laid-Open Patent Applications 58-82788 and 60-13852; a metallic salt of benzoic acid, as disclosed in Japanese Laid-Open Patent Application 61-47292; and a substituted salicylic acid compound, as disclosed in Japanese Laid-Open Patent Application 62-169681. However, the fastness to fats and oils, plasaticizers, and water of the images recorded on the thermosensitive recording materials comprising the above-mentioned color developers is still insufficient.
The formation of an intermediate layer is proposed to increase the recording sensitivity of the thermosensitive recording material. For instance, there are proposed an intermediate layer mainly comprising a variety of inorganic pigments, and an intermediate layer comprising non-expandable void particles. However, these recording materials have the shortcomings that the thermal coloring sensitivity is insufficient and the recording properties are poor.